Disable skills in guardian review sessions

[dylan-hurd-oai]

Summary

• force guardian review session configs to disable skills developer-message injection
• add a guardian config regression test
• add a suite-level guardian-mode integration test that verifies parent requests still include skills while guardian requests do not

Stack

Base branch: dh--agent-spawn-skills
Depends on: #18444

Tests

• cargo test -p codex-core guardian_mode_does_not_inject_skills_message
• cargo test -p codex-core guardian_review_session_config
• just fix -p codex-core